Protecting device for electrical circuits



0. DREYER PROTECTING DEVICE FOR ELECTRICAL CIRCUITS Fll'd M1- 26. 1921 Patented a. 28, 1924.

UNITED STATES PAT ENT OFFICE.

OTTO DREYER, OF ESSEN-ON-THE-RUHR, GERMANY.

PROTECTING DEVICE FOR ELECTRICAL CIRCUITS.

, Application filed August 26, 1921.

. a citizen of the German Republic, residing at Essen-on-the-Ruhr, Rhenish Prussia, Germany, have invented certain new and useful Improvements in a Protecting Device for Electrical Circuits, of which thefollowing is a specification.

My invention refers to means for protecting electric machinery or instruments against overheating due to slight but prolonged overloading.

For the protection of electric machinery and other electric devices and instruments against overheating there are used, on the one hand, the so-called fuses, which consist breakers.

of a wire which is traversed by the current and which melts When a certain strength of current is exceeded, owing to the heat produced in it by the current, and thereby cuts off the current immediately. Fuses of this nature involve the disadvantage that they generally possess a far smaller capacity for heat than the machine to be protected, and they must therefore be dimensioned in such manner that they can carry a large overload for a short time, for example, when starting the machine. In consequence thereof they do not afford efiicient protection against overheating of the machine due to a smaller but prolonged overload.

On the other hand, devices are also known which are actuated not by the heat produced by the current in the device itself but by the heat of the object to be protected and for this purpose they are combined either in the object itself to be protected, or in an auxiliary coil having the same heat-constant. Such devices, which consist, for example, of a contact-thermometer or a soldered joint of easily fusible metal are hereinafter called temperature switches in contra-distinction to the aforesaid fuses.

The well known temperature switches, although protecting the circuit against overheating, nevertheless involve the disadvantage that the cutting off of the current can only be effected with certainty when the current is small. With large currents (for example in motors) these temperature cut-outs have therefore been used in combination with a rela for releasing automatic circuit expensive and often cannot be employed at for them.

hese devices are however very Serial No. 495,845.

These drawbacks are obviated according to 'the present invention by combining a temperature switch with a fuse in such a manner that only the beginning of the circult breaking process is effected by means of the temperature switch while the actual completion of the operation is effected by means of the fuse. In this manner a far simpler and at the same time reliable protection against overheating even with large currents is obtained. The temperature switch can either be so constructed that it conducts current itself or it can co-act mechanically with a vcontact, which, when the temperature switch comes into operation, causes an increased current to traverse the fuse and thus causes it to melt. Finally the temperature switch can either be itself combined with the apparatus to be protected or it can be surrounded by a special auxiliary heating coil. In the latter case the coil together with the two safety devices is suitably fitted in a common casing or housing. The temperature switch can be advantageously constructed as a movable magnetic core so that it can be influenced from one and the same coil not only by its heat but also by its electromagnetic action in a manner similar to automatic circuit-breakers.

Two constructional embodiments of the invention are illustrated by way of example in the accompanying drawing in which Fig. 1 shows the diagram of connections for a form of construction in which the temperature switch acts as a conductor of gurrent and is arranged in parallel with the use,

Fig. 2, discloses a specific embodiment of the protective device which is illustrated diagrammatically in Fig. 1,

Figs. 3 and 4, show a temperature switch belonging thereto in its soldered and unsoldered condition,

Fig. 5 is a diagram of connections for a form of construction in which the temperature switch does not conduct current but co-acts mechanically with an auxiliary contact,

Fig. 6, is a longitudinal section of a plug constructed in accordancewith Fig. 5 and Fig. 7, is a cross section on the line A-A of Fig. 6.

In the diagram of connections illustrated in Fig. 1, 1 is the coil to be protected or an auxiliary coil of which the temperature is dependent on that of the former; 2 is the temperature switch and 3 the ordinary fuse. The temperature switch 2 is connected in a parallel with the fuse 3 and-is formed by" a soldered joint of easily fusible metal which I offers only a small resistance'to the passage of the current. Normally, therefore, nearly 31 through t the whole of the current flows through the soldered joint 2 and only a small portion he fuse 3'. The latter is thus shortcircuited to a certain extent by the soldered joint. The soldered joint 2 must be'fitte-d in close proximity to the coil 1 or even in the .coil itself so that it is heated by it and betakes place without arcing. As the drop in potential across an ordinary fuse is only small, an appreciable spark cannot be formed if the correct dimensions be chosen Fig. 2 shows, by way of example, a con-. structional embodiment of a protecting device which is arranged in accordance with the diagram illustrated in Fig. 1. The device consists of a casing 5 of porcelain or other suitable material, provided with a removable cover 1. In the interior of the casing there is arranged the coil 1 and an interchangeable fuse3, which, in the example illustrated is shown as a Diazed plug. The temperature switch is formed by a sleeve 6,

which is interchangeably held in the coil 1,

diagram shown in Fig. 1. The current enters through terminal 9 and is conducted to the lower end of the coil 1 and from the upper end of this coil it passes, on the one hand, to the contact spring 10 and the fuse 3 to the return terminal 11, and, on the other hand, through the ring 12 and the sleeve 6 through the soldered joint 2 on the bolt 8 and thence through the lower contact spring 14 back again to the terminal 11. The coil 1 is of such dimensions that it possesses approximately the same heat-constant as the coil to be protected, so that if the latter exceeds a certain temperature, the soldered joint 2 is brought to the softening point by the heat of the coil 1. The bolt 8, owing to the action of the spring 7, then moves upward into the position shown in Fig. 'and is thus brought out of contact with the spring 14; so that this circuit is broken and the whole of the current passes through the fuse 3, which therefore melts and cuts off the current entirely.

In order to be able to use the device again it is only necessary to remove the cover 4, to change the fuse plug 3 and to replace the sleeve 6 by a new sleeve (as shown in Fig. 321. In order that the sleeve 6 be easily exc angeable, it is provided at its upper end with a funnel shaped split or slotted head 15 which is pressed against the ring 12 and thus forms the contact. 4

In the example illustrated in Fig. 5 the fuse 3 is not directly shortwircuited by the soldered joint 2 but by means of a small auxiliary switch 20 which is opened by the spring actuated bolt 8 when the soldered joint softens and thereby breaks the shortcircuit section which is connected in parallel with fuse 3, so that all the current flowing through the coil 1 now passes through the fuse 3. The spring-actuated bolt 8 together with its soldered joint is so fitted to a magnetic core movably arranged in the coil that.

when there occurs a strong impulse of current the entire core together with the bolt 8 is electro-magnetically forced against the auxiliary switch and thereby disconnects it. A constructional embodiment of a protecting device of the latter kind in the form of a plug for movable cables is specifically illustrated in Figs. 6 and 7 In these Figures, 1 is again the coil, 2 the soldered joint which forms the temperature switch, 3 the fuse and 20 the auxiliary switch which is connected in parallel with the latter. This switch consists of two areshaped spring arms 21, which in the switched-on position shown in Figs. 6 and 7 -are pressed apart by the pair of levers 22 and thereby pressed against the contact springs 23 which are connected with the ends of the fuse 3.

The device for opening the auxiliary switch 20 consists of a magnetic core 25 movably arranged in the coil 1 and generally held by means of a weak spring 24 in the lower position shown in Fig. 6. The upper end of the magnetic core 25 is formed as a slender pin 26 the end plate 27 of which is applied from below against the head 28 of a non-magnetic sleeve 29. The latter is under the action of the spring 30 which tends to force the sleeve upwards but it normally prevented from doing so by the soldered joint 2. When a given temperature is exceeded the coil 1 causes the soldered joint 2 to soften and the sleeve 29 under the action of the spring 30 forces the pair of levers 22 upwards. The spring arms 21 move inwards and the contact with the springs 23 is broken so that the fuse 3 is now blown.

The device illustrated in Figs. 6 and 7 acts not only when a given temperature is exceeded but also if a sudden inadmissibly great current occurs, since the magnetic core electro-magnetic action of the current and thus also presses upwards the pair of levers and opens the auxiliary switch. In this manner, particularly in the case of a short circuit, a quick break, is effected before in.-

3 jurious heating up of the coil can take place.

In order to set the protective device in action again after t e electro-magnetic switching action it is only necessary to turn the cover 31 about the hinge 32, to remove the burnt out fuse 3 and to press back the auxiliary switch 20 into the switched-on position as illustrated in Fig. 6 b means of a rod or pin inserted through t e opening .33, after which a new fuse can be 1nserted. Owing to the arrangement of the fuse 3 above the aperture 33 which gives access to the auxiliary switch, a locking of this branch of the circuits is obtained in so far that the re-insertion of the auxiliary switch 20 is only 'ble when the fuse 3 has been removed. joint 2 also melt owing to overheating, this part of the device must of course be exchanged as well.

For protection against mechamcal in ury the whole device can be enclosed in a metal casing 36.

. I wish it to be understood that I do not desire to be limited to the exact details of construction shown and described, for obsoldered joint being adapted to become unsoldered by an increase of temperature in the protected circuit and to release said switch. 1

2. In a protective device for an electric circuit, in combination: an ordinary fuse, a switch connected in parallel with the fuse and comprising a pair of toggle-levers normally stretched and in this position holding the switch in its switched-in position, a sup-port for the toggle-joint of said pair of toggle-levers held in its normal position by a soldered joint, and a spring tending to lift said support, said support being adapted on the one hand to secure said pair of toggle-levers in its stretched position, and on the other hand to bend said togglelevers when the soldered joint becomes unsoldered by an increase of temperature in the rotected circuit, the levers thereby opening said switch,

Should the soldered 3. In a protective device for electric circuits, in combination: an ordinary fuse, a temperature switch controlling the current traversing the fuse,.and a special coil surrounding said temperature switch, said coil being arranged to become heated to the same degree as the circuit to be protected, the connection between said fuse and said switch being such that, on said switch being actuated, the current traversing said fuse is automatically increased. a

4. In a protective device for electric circuits, in combination: an ordinary fuse, a.

temperature switch controlling the current traversing the fuse, and a special coil surrounding said temperature switch the latter having the form of an electro-magnetic switch influenced by the electro-magnetic action of said-coil.

5. In a protective device for electric circuits, in combination: an ordinary fuse, a switch controlling the current traversing the said fuse, a spring actuated operating member for said switch, a soldered joint holding said member in its inoperative position, a magnetic core carrying said operating member and movable in the opera'tin direction of the latter, and a special coil arranged to become heated to the same degree as the circuit to be protected, said coil surrounding said soldering joint as well as said magnetic core. 4

6. In a protective device for electric circuits, in combination: an ordinary fuse, a switch connected in parallel with said fuse and comprising a pair of toggle-levers normally stretched and in this position holding said switch in its switched-in position, a movable support for the toggle-joint of said levers, a spring tending to lift said support. a magnetic core carrying said support and spring and movable in the same direction as the support, a soldered joint between said support and said magnetic core holding said support in its normal position, a spring holding said magnetic core in its normal position, and a special coil arranged to become heated to the same degree as the circuit to be protected and surrounding the said support as well as the said magnetic core, said coil being adapted on the one hand by an increase of temperature, to unsolder the said soldered joint so as to allow the support to be lifted, and on the other hand, by its increasing electro-magnetic action, to lift said magnetic core together with said support for bending said toggle-levers and thereby opening said switch.

7. In a protective device for electric circuits, in combination: an ordinary fuse, a temperature switch controlling the current traversing said fuse and comprising a soldered joint adapted to become unsoldered by an increase of temperature in the protected circuit, a common casing having an opening and surrounding said fuse and said switch, the said fuse being arranged between said switch and the opening of said casing thereby allowing the soldered joint of said switch to be replaced only after said fuse has been removed.

8. In a protective device for electric cir cuits in combination, an ordinary fuse, a shunt circuit parallel to said fuse and carrying most of the current, and a temperature switch adapted to normally close said shunt circuit, but, on being operated, to open said circuit, so asto increase the current traversing said fuse.

In testimony whereof I afiix my signature.

ocrro DREYE-R. 

